Layer-by-layer Synthesis and Transfer of Freestanding Conjugated Microporous Polymer Nanomembranes

Summary

In this paper we describe the interfacial synthesis of conjugated microporous polymers (CMP) on sacrificial substrates, and the dissolution of the substrate for the preparation of freestanding CMP nanomembranes. In addition, we will describe how the fragile nanomembranes can be transferred to other substrates.

Abstract

CMP as large surface area materials have attracted growing interest recently, due to their high variability in the incorporation of functional groups in combination with their outstanding thermal and chemical stability, and low densities. However, their insoluble nature causes problems in their processing since usually applied techniques such as spin coating are not available. Especially for membrane applications, where the processing of CMP as thin films is desirable, the processing problems have hindered their commercial application.

Here we describe the interfacial synthesis of CMP thin films on functionalized substrates via molecular layer-by-layer (l-b-l) synthesis. This process allows the preparation of films with desired thickness and composition and even desired composition gradients.

The use of sacrificial supports allows the preparation of freestanding membranes by dissolution of the support after the synthesis. To handle such ultra-thin freestanding membranes the protection with sacrificial coatings showed great promise, to avoid rupture of the nanomembranes. To transfer the nanomembranes to the desired substrate, the coated membranes are upfloated at the air-liquid interface and then transferred via dip coating.

Introduction

The preparation of ultra-thin polymer membranes is of high interest for applications in gas separation and nanofiltration. Challenges in the synthesis are represented by (a) the control of the membrane thickness and the homogeneity and (b) transfer of such fragile membranes. To overcome challenge (a), molecular layer-by-layer synthesis¹ has shown great promise in controlling the thickness and homogeneity of thin films grown at the solid-liquid interface.^2,3 Controlling the number of layers linearly controls the film thickness. The l-b-l method has been successfully used to fabricate surface mounted metal organic frameworks (SURMOFs),^4-7

Protocol

1. Synthesis of CMP Thin Films through Sequential Addition

1. Self-assembled monolayer (SAM) functionalization of gold on mica.
   1. Prepare 1 mM solution of 11-thioacetyl-undecane acid-propargyl amide¹⁴ in ethanol (SAM-solution). Mix using ultrasonic bath till solution is clear. Protect the bottle from light using aluminum foil.
   2. Obtain gold coated mica wafer under argon. After withdrawal from the storage container immerse the mica wafer directly to the SAM-solution for 18 hr.
   3. Take the prepared Au-mica wafer out of the SAM-solution, rinse with ethanol and dry under nitrogen stream. Afterwards store the substrate....

Results

The membranes are characterized by infrared reflection absorption spectroscopy (IRRAS).¹⁶ Figure 4 shows IRRA-spectra from a CMP-membrane transferred to a gold wafer. Typical bands from the vibrations of the aromatic backbone are at 1,605 cm^-1, 1,515 cm^-1 and 1,412 cm^-1. Unreacted alkyne and azide groups can be observed by characteristic bands at 2,125 cm^-1 and 1,227 cm^-1. Figure 5 shows a scanning electron microscopy (SEM.......

Discussion

For the synthesis of the CMP-film the solution of the catalyst has to be fresh. A broken catalyst (i.e., oxidized) is indicated by a blue coloration of the solution. The fresh solution is colorless.

A crucial point is to cut the edges of the mica substrate after spin coating PMMA. Also defects in the substrate should be cut, i.e., each spot were the PMMA can come in contact with the mica substrate, because of a missing gold layer. Otherwise the gold layer cannot be stripped o.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Acetone	VWR BDH Prolabo	20066.330	AnalR NORMAPUR
Potassium iodide	VWR BDH Prolabo	26846.292	AnalR NORMAPUR
Ethyl acetate	VWR BDH Prolabo	23882.321	AnalR NORMAPUR
Tetrahydrofuran (THF)	VWR BDH Prolabo	28559.320	HiPerSolv CHROMANORM
THF waterfree	Merck Millipore	1.08107.1001	SeccoSolv
Iodine	Sigma-Aldrich	20,777-2
Tetrakis(acetonitrile) copper(I) hexafluoro-phosphate	Sigma-Aldrich	346276-5G
Poly(methyl methacrylate) 996 kDa (PMMA)	Sigma-Aldrich	182265-25G
1.1.1.1 Methanetetrayltetrakis(4-azidobenzene) (TPM-azide)			Provided by AK Prof. Bräse. Institute of organic chemistry, Karlsruhe Institute of Technology. Synthesized according to 9.
1.1.1.1 Methanetetrayltetrakis(4-ethinylenebenzene) (TPM-alkyne)			Provided by AK Prof. Bräse. Institute of organic chemistry, Karlsruhe Institute of Technology. Synthesized according to 9.
11-thioacetyl-undecaneacid propargylamide			Provided by AK Prof. Bräse. Institute of organic chemistry, Karlsruhe Institute of Technology. Synthesized according to 8.
gold/titan coated silicium-wafer	Georg Albert PVD, 76857 Silz, Germany
gold coated mica	Georg Albert PVD, 76857 Silz, Germany